Tandy 1000 HX Technical Reference Manual page 396

Function Descriptions:
Set CRT Mode
Entry Conditions:
AH = 0
AL = mode value, as follows:
Alpha Modes
AL = 0: 40x25 black and white
AL = 1: 40x25 color
AL = 2: 80x25 black and white
AL = 3: 80x25 color
Graphics Modes
AL = 4: 320x200 color graphics
AL = 5: 320x200 black and white graphics with 4
shades
AL = 6: 640x200 black and white graphics with 2
shades
AL = 7: Reserved
Additional Modes
AL = 8: 160x200 color graphics with 16 colors
AL = 9: 320x200 color graphics with 16 colors
AH = A: 640x200 color graphics with 4 colors
Note: If the high order bit of the AL register is 1 then
the video buffer is not cleared.